1. Field of the Invention
This invention relates generally to the radiofrequency (RF) electromagnetic field, denoted as B1, used for magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) spectroscopy instruments. Particularly, the invention relates to the use of materials with high dielectric constant (HDC) or permittivity with low electrical conductivity for RF field generation in the MRI or NMR instruments.
2. Description of the Prior Art
RF coils are used to produce RF magnetic field (B1) for excitation and detection of the magnetization signal from an object such as parts of human body during MRI and MRS data collections. The RF field can be generated by a coil or a set of coils. The RF field is transmitted into the sample or human body to excite the nuclear spins. Subsequently, the RF signals from the nuclear spins are received by the same or a different set of RF coils. The present invention improves the efficiency of RF coils for both transmission and reception of the B1 field. The efficiency of a RF coil includes B1 field uniformity and intensity in a region of interest (ROI) produced by unit input current. In general, a stronger B1 field generated by a RF coil with unit input current translates to better reception sensitivity and lower RF heating of the tissue samples that may be hazardous to human body when it reaches to a certain level. The reception sensitivity of a RF coil is experimentally determined by measured image signal to noise ratio (SNR) from an ROI obtained with standardized imaging protocols, and the heating effect by RF field can be assessed numerically with calculation of Specific Absorption Rate (SAR) distribution in the human body (1).
The SNR of an MR image is critically important for the quality of the image as MRI is an intrinsically low SNR instrument. Significant amount of efforts and resources have been invested in order to gain higher image SNR in a given image data acquisition time. In particularly, this includes the use of higher static magnetic field strength from current 1 to 1.5 Tesla to 3 to 7 Tesla, which is extremely expensive. Unfortunately, an increase in static magnetic field strength leads to a higher frequency of RF field, which, in turn, dramatically increases RF heating effect of the tissue (1) and creates RF field inhomogeneity artifacts (2-5). These two problems pose serious challenges for high field MRI development in human imaging. The present invention introduces a novel method of use of HDC materials that increases B1 field intensity inside the sample and reduces SAR in the sample during image acquisition.
The recent experimental data in high field (3-8 Tesla) human imaging systems demonstrated that high dielectric constant of human body plays an important role for RF field behavior in human body in high field. The electrical properties, geometry, and relative position of the sample in the coil become important factors in determining the B1 field distribution inside the sample (6-11). Consequently, adjustment of B1 field distribution inside the sample or human body and the coupling between the sample and coil can be facilitated with HDC materials. Foo, et al. proposed a method of correcting for the RF inhomogeneity in human body observed on a 4 Tesla MRI system by “dielectric loading of the coil-to-shield space in an RF resonator (coil and shield assembly)” (10). Based on Foo's theoretical analysis, he proposed to adjust the RF homogeneity by loading the coil-to-shield space with dielectric material of suitable relative permittivity so as to alter the propagation constant of the coil. With theoretical calculations, Foo predicted that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil-to-shield space would reduce the RF field inhomogeneity from +/−15% to about +/−3% over a central 30-cm-diameter region of a homogeneous 40-cm-diameter body at both 64 MH and 170 MHz corresponding to a 1.5 and 4 Tesla MRI system respectively. However, their experimental results at 4 Tesla showed that “the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity.” There at least three distinctive differences in Foo's work from the present invention. 1) The dielectric material is inserted in the coil-to-sample space in the present invention, while it was loaded in “the coil-to-shield space” in Foo's work. In fact, the dielectric material is placed in the opposite side of the RF coil of the present invention. As demonstrated in their experimental results, Foo's approach produced totally opposite outcome i.e. a “increased RF power requirements and reduced coil sensitivity”. 2) The theoretical bases are totally different. In the present invention, the HDC material is used to couple the RF field produced by the RF coil with the sample. In Foo's work, the dielectric material was used to manipulate “the axial propagation constant of RF resonator” itself. 3) In Foo's work, the choice of dielectric material was based on the specific design and geometry of the RF coil or resonator. In principle, the present invention requires no knowledge of the RF coil configuration. It is more effective, however, that HDC-pads of the present invention are developed to fit a specific coil design for optimal effect.
To address the same RF field inhomogeneity issue in the head image taken at 4 Tesla, Alsop et al. presented a novel spiral RF volume coil design for high field MRI use (2). Images acquired with his spiral coil design showed a signal drop on the top of the human head. It was attributed as the abrupt change in dielectric property between tissue and air at the top of head since his coil design theoretical analysis was based on a mathematical model of an infinite long cylinder. To mitigate this additional problem associated with the spiral coil design, a dielectric pad at the end of the coil was included during image data collection. In his work, the introduction of a dielectric pad was specifically used only for compensation of the signal drop caused by the use of spiral coil. In the present invention, HDC-ads are placed inside an RF volume coil of any design to improve the efficiency of a given RF coil.